Imaging devices such as inkjet printers typically operate one or more printheads that are configured to eject ink for marking media. In direct marking printers, the ink is applied directly on the media, rather than to an intermediate printing surface. The media can be, for example, a surface of a continuous web of media material, a series of media sheets, or other surfaces that are desirably marked. A printhead controller typically controls the one or more printheads by generating a firing signal with reference to image data.
In order for printed images to correspond closely to the image data, both in terms of fidelity to image objects and colors represented by the image data, the printheads are registered with reference to an image receiving surface of the media and with the other printheads in the printer. Registration of printheads refers to a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the relative positions of the printheads with reference to the imaging surface and with reference to the other printheads in the printer.
Two or more printheads can be mounted linearly, or in other configurations, to a support structure, to form an array of printheads. Not only is registration between individual printheads important, but control of the registration of the supporting structure with respect to the image receiving surface is also desirable. A distance between the printheads and the imaging surface is carefully selected to optimize the imaging process. If the gap is too small, burnishing of the printheads can occur when the image receiving surface contacts the face of the printheads. Burnishing not only reduces the life of the printheads, but results in poor image quality, unintentional markings, and increased downtime of the printer during maintenance. If the gap is too large, image quality suffers, particularly in high speed printers, where a large gap can result in decreased accuracy of the ejected drops forming the printed image. A nominal gap distance between printheads and an image receiving surface can be, for example, about 1 mm or less.
The setting of a proper gap between a printhead and an image receiving surface is important where a printer is designed to accept a variety of imaging surfaces, including surfaces having a tendency to wrinkle, having different thicknesses, or having uneven surfaces. When the gap distance is small, even small variations in the height of the media can cause the media to impact and damage a printhead. Consequently, detecting the gap distance of the printhead array from the imaging surface and adjusting the gap distance appropriately are important considerations for image quality and printer operation. For example, media for a continuous feed printer can include seams where rolls of the media are spliced together, resulting in thicker sections of media that may not lie flat.
While vacuum belts and other hold-down systems, such as electrostatic hold-down systems, have been used to hold media flat against a surface, such systems are not usable to adjust the gap distance between the media and the printhead. In other words, while a hold-down system might prevent a wrinkle from impacting the printhead by flattening out the wrinkle, a hold-down system would be ineffective when a height of the media increases, such as with a seam.
Previously known printers include an endless belt media transport with a vacuum hold-down system that holds a media flat against the endless belt as the media passes a printhead array. The printer also includes a sheet height sensor that determines whether a height of the media exceeds a threshold height with regard to a gap distance between the media and the printhead array. The printhead array and/or the media transport can be repositioned via actuators in response to a signal from the sheet height sensor in order to maintain a sufficient gap distance so that the media does not impact the printhead.
Moving the printhead array or the media transport supporting the media, even only briefly before returning to the nominal gap distance, however, introduces potential error with respect to registration of the printheads and the imaging surface. Even if relative motion between the media and the printhead array is slight, the printhead array must be reregistered in order to enable the printer to produce an accurate printed image. This process is repeated each time the gap distance needs to be adjusted, which may interrupt or delay the printing process. Actuating the media or the printhead array also requires significant portions of the printer to be on actuated frames which can be complex and expensive, as well as providing additional points of failure in the printer. Therefore, printhead protection that compensates for media surface variations without disturbing registration of the printhead would be useful.